Protecting an element from excessive surface wear by localized hardening

ABSTRACT

A method of protecting an element from excessive surface wear is provided. In this method, a localized area of the element that will be subjected to excessive surface wear is exposed to induction heating for a period sufficient to heat the localized area to an elevated temperature at which the localized area undergoes austenitic transformation. The localized area is quenched, followed by tempering, and then cooling. A result of the method is a localized hardened area formed monolithically with the element and having a localized hardness that is greater than a base hardness of the element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of Provisional Application No. 61/076836, filed Jun. 30, 2008, the disclosure of which is ENTIRELY incorporated herein by reference.

FIELD

The invention relates generally to methods for hardening elements that are subjected to excessive surface wear during use. The methods are particularly suitable for use with elements such as downhole tools, e.g., drill bits, tool joints, stabilizers, and drill collars, and other elements that require protection against excessive surface wear.

BACKGROUND

FIG. 1 shows a conventional downhole tool string 31 suspended in a wellbore by a derrick 32. The downhole tool string 31 is coupled to a data swivel 34 that connects to surface equipment 33, such as a computer. The data swivel 34 is adapted to transmit data to and from a transmission network integrated with the downhole tool string 31 while the downhole tool string 31 is rotating. The integrated transmission network comprises the transmission systems of the individual downhole components (e.g., components 36, 57, 35) of the downhole tool string 31. Preferably, the downhole component is a drill pipe 57 or a tool 35. One or more tools 35 may be located in the bottom hole assembly 37 or along the length of the downhole tool string 31. Examples of tools 35 in a bottom hole assembly 37 comprise sensors, drill bits, motors, hammers, and steering elements. Examples of tools 35 located along the downhole tool string 31 are links, jars, seismic sources, seismic receivers, sensors, and other tools that aid in the operations of the downhole tool string 31. Different sensors are useful downhole, such as pressure sensors, temperature sensors, inclinometers, thermocouples, accelerometers, and imaging devices. Downhole tools 35 consisting of tubulars configured with sources and sensors are commonly referred to as drill collars. The illustrated downhole tool string 31 is a drill string.

During operation, the downhole tool string 31 often encounters extreme conditions, such as high heat, high pressure, torsion and tension-compression stress, vibration, and impact. The components of the downhole tool string 31 are also subjected to contact with abrasive formations, erosive fluids, frictional contact with other tool elements, and sources of wear. To protect against these conditions, particularly excessive wear, various elements of the downhole tool string 31 are typically provided with a welded metal hardfacing or hardface coating. These hardfacing coatings provide hardness to the exterior of the string elements, particularly the surfaces that will come in contact with the abrasive formations. The required hardness is often accomplished by providing a coating composed of tungsten carbide particles, which are cemented in place by a metal binder. The matrix formed by the carbide particles and the binder is applied as a coating to the various surfaces.

Although conventional hardfacing techniques have proven useful in the industry, a need remains for improved techniques to harden tool and element surfaces, particularly for subsurface applications.

SUMMARY

Aspects of the invention include a method of protecting an element from excessive surface wear. The method comprises exposing a localized area of the element that will be subjected to the excessive surface wear to induction heating for a period sufficient to heat the localized area to an elevated temperature at which the localized area undergoes austenitic transformation. The localized area is quenched after the exposure, followed by tempering, and then cooling. A result of the method is a localized hardened area formed monolithically with the element and having a localized hardness that is greater than a base hardness of the element.

Aspects of the invention include a downhole tool protected from excessive surface wear. The downhole tool comprises a tool body made of a material having a base hardness and a localized hardened area formed monolithically with the tool body. The localized hardened area has a localized hardness that is greater than the base hardness.

Other aspects and advantages of the invention will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, described below, are referenced in the background and detailed description. The figures are not necessarily to scale, and certain features and certain views of the figures may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

FIG. 1 is a schematic of a conventional drill rig showing a drill string and a system for drilling a well bore in a subsurface formation and obtaining formation measurements.

FIG. 2 is a schematic of a downhole tool tubular implemented with a hardened segment in accordance with aspects of the invention.

FIG. 3 is a cross-section of a pin-end tool joint including a localized hardened area.

FIG. 4 is a schematic of induction heating of an element.

FIG. 5 is a cooling curve analysis comparing cooling rates of different quenchants.

DETAILED DESCRIPTION

Aspects of the invention entail processes to produce an induction hardened surface or area on a desired element. For purposes of this disclosure it will be clearly understood that the word “element” means any type of tool, machine, apparatus, or component that may be subjected to excessive wear during operation. According to aspects of the invention, an element, e.g., a downhole tool joint, is hardened by a process entailing exposure to a surface heat treatment, followed by polymer quenching, low-temperature tempering, and air cooling.

FIG. 2 shows a drill pipe 57 including a tubular 55, a pin-end tool joint 14, and a box-end tool joint 15. Typically, the drill pipe 57 is made of metal, e.g., low-alloy steel. The pin-end tool joint 14 and box-end tool joint 15 are attached to either ends of the tubular by a method such as friction/inertia welding. The tool joints 14, 15 have thickened walls in comparison to the tubular 55 in order to accommodate mechanical and hydraulic tools used to connect and disconnect the drill pipe 57 from a tool string, such as the downhole tool string 31 shown in FIG. 1. Similar tool joints are found on other downhole components that make up a downhole tool string. The tool joints 14, 15 may have a smaller inside diameter and a larger outside diameter in order to achieve the thicker walls. Therefore, it is typically necessary to forge, or “upset”, the ends of the tubular 55 in order to increase the wall thickness of the tubular 55 prior to attachment of the tool joints 14, 15. The upset end portions 19, 20 of the tubular 55 provides a transition region between the tubular 55 and the tool joint 14, 15, respectively, where there is a change in the inside and outside diameter of the drill pipe 57. High torque threads 16 on the pin-end 16 and on the box-end 17 (the threads are internal at the box-end 17) provide for mechanical attachment of the drill pipe 57 in a downhole tool string.

Aspects of the invention include forming a localized hardened area 62 on the drill pipe 57. In the embodiment shown in FIG. 2, the localized hardened area 62 is formed on the pin-end tool joint 14 of the drill pipe 57. By “localized,” it is meant that the hardening is confined to the area 62 shown and that the base hardness of the material of the pin-end tool joint 14 (or element in general) is not significantly affected by the forming of the hardened area 62 on the pin-end tool joint 14 (or element in general). The localized hardened area 62 is monolithic with the pin-end tool joint 14. By “monolithic” it is meant that the localized hardened area 62 is formed in-situ on the pin-end tool joint 14 by manipulating the material of the pin-end tool joint 14 rather than by applying an external hardened or hardening material on the pin-end tool joint 14. By “localized hardened,” it is meant that the area 62 has a localized hardness that is greater than the base hardness of the material in which it is formed.

FIG. 3 shows a cross-section of the pin-end tool joint 14. The depth of the localized hardened area 62 into the wall of the pin-end tool joint 14 is indicated at “d.” In certain aspects of the invention, d is at least 0.25 in. (0.635 cm). In other aspects of the invention, d is in a range from 0.25 in. (0.635 cm) to 0.375 in. (0.9525 cm). Although the localized hardened area 62 is formed on the pin-end tool joint 14 in the embodiment shown in FIGS. 2 and 3, it should be clear that this is not intended to limit the invention as such. The localized hardened area 62 can be formed on any element deserving of protection from excessive surface wear. Aspects of the invention include forming the localized hardened area 62 on elements or components of a downhole tool string, where the localized hardened area 62 will be monolithic with the respective downhole element or component on which it is formed.

Aspects of the invention include a process for forming a localized hardened area on an element. The process includes exposing a localized area of the element that will be subjected to excessive surface wear to localized heat for a period sufficient to heat the localized area to an elevated temperature at which the localized area undergoes austenitic transformation. Preferably, the elevated temperature is such that the localized area undergoes 100% austenitic transformation. In a preferred embodiment, the localized heat is provided by an electromagnetic induction heater.

Referring to FIG. 4, a localized area 70 of an element 72 (e.g., a tool joint) is disposed adjacent to an electromagnetic heater 74. The induction heater 74 includes a susceptor 76 and induction coil 78. A magnetic field is generated by running alternating current through the induction coil 78. The magnetic field induces eddy currents in the susceptor 76 to generate the localized heat. Typically, the susceptor 76 is concentric with the induction coil 78, and the induction heater 74 delivers heat uniformly and locally to the localized area 70. The induction heater 74 heats the localized area 70 to the elevated temperature required for austenitic transformation. In certain aspects of the invention, the induction heater 74 operates with frequency ranges from 1 KHz to 10 KHz and power densities on the order of 0.5 KW/in² to 10 KW/in². In general, the operating parameters of the induction heater 74 will be chosen to achieve the goal of heating the localized area 70 to achieve austenitic transformation in the localized area 70. To achieve austenitic transformation, the element 72, at least in the vicinity of the localized area 70, is made of an austenitizeable material. In certain aspects, the austenitizeable material is a low-alloy steel.

The austenitizing temperature is determined based on the chemistry of the austenitic material. For low-alloy steel, the austenitizing temperature (T) can be determined according to the following expression:

$\begin{matrix} {T = {910 - {203 \times \sqrt{C}} + {44.7 \times {Si}} - {15.2 \times {Ni}} + {31.5 \times {Mo}} + {104 \times V} + {13.1 \times {W\left( {{^\circ}{\mspace{11mu} \;}{C.}} \right)}}}} & (1) \end{matrix}$

where C represents percent weight (wt %) of carbon in the steel, Si represents wt % of silicon in the steel, Ni represents wt % of nickel in the steel, Mo represents wt % of molybdenum in the steel, V represents wt % of Vanadium in the steel, and W represents wt % of Tungsten in the steel.

The elevated temperature to which the localized area 70 is heated for austenitic transformation may be the same as the calculated austenitizing temperature or may be higher than the calculated austenitizing temperature. In certain aspects, the elevated temperature is the calculated austenitizing temperature plus a safety margin to ensure 100% austenitic transformation. In certain aspects, the austenitizing temperature is at least 1525° F. (830° C.). In certain aspects, the safety margin may be, for example, about 37.8° C. (100° F.) above the calculated austenitizing temperature. The advantage of localized heating, preferably by use of electromagnetic induction, is that the hardening can be confined to the localized area 70, leaving the remainder of the element 72 substantially unchanged.

After the austenitizing heat treatment, the localized area 70 is quenched in a liquid medium. The objective of quenching the high strength low alloy (HSLA) steel in a liquid medium is to remove the heat from the part as quickly as possible to change the crystal structure from the austenitic state (FCC-face center cubic) to a martensitic state (BCT-body center tetragonal). In a preferred embodiment, the liquid medium is an aqueous polymer medium. In some embodiments, the aqueous polymer medium is comprised of a low concentration of a polymer and water. In some embodiments, the polymer is selected from polyalkylene glycol and polyethylene glycol. Quenching in a controlled aqueous polymer medium helps reduce residual stress and prevent cracking. In certain aspects, quench bath temperature is between 90° F. (˜32° C.) and 105° F. (˜41° C.).

AQUA-QUENCH 245 is an advanced biostable polymer quenchant suitable for induction heat treating. It is a polyalkylene glycol based polymer quenchant formulation with a combination of ingredients that provide greater stability to microbial intrusion of the quenchant. AQUA-Quench 245 is specifically designed for use in induction hardening and immersion quenching applications. FIG. 5 is a cooling curve analysis showing the cooling rates of a steel quench in an aqueous polymer medium. The quenchant for Sample #1 was 20 vol % AQUA-QUENCH 365 and 80 vol % water. The quenchant for Sample #2 was 20 vol % AQUA-QUENCH 245 and 80 vol % water. The quenchant for Sample #3 was 17 vol % AQUA-QUENCH 245 and 83% water. From FIG. 5, both AQUA-QUENCH 245 solutions (#2 and #3) are faster than the AQUA-QUENCH 365 (#1) in cooling the steel parts at high temperatures. Cooling rates for the different quenchants below 700° F. (˜371° C.) are similar. Both AQUA-QUENCH 245 solutions will provide deeper hardening of the quenched part with the same level of distortion control as the AQUA-QUENCH 365. AQUA-QUENCH 365 is a concentrated solution of polyethylene glycol. In the examples shown in FIG. 5, the bath temperature was held at 100° F.

Although AQUA-QUENCH 245 and AQUA-QUENCH 365 have been presented as examples of polymer quenchants, it should be clear that aspects of the invention are not limited to these particular polymer quenchants. Other types of polymer quenchants may be used. In some cases, non-polymer quenchants may also be used provided that the quenchant is capable of producing the desired hardness in the material being hardened, preferably without cracking the material.

After quenching, the localized area 70 is tempered at a low temperature. In certain aspects, the low temperature is in a range of 275° F.±25° F. Tempering time is selected to attain a specified case hardness range for the localized area while maintaining a desired base hardness for the element. As an example, tempering time may be on the order of 2 hours. In certain aspects, the minimum tempering time is 2 times the wall thickness of the element. After tempering, the localized area 70 is cooled, e.g., by air cooling.

The resultant product of the process described above is a localized hardened area monolithically formed in an element, where the localized hardened area has a localized hardness that is greater than the base hardness of the element. In certain aspects, the element 72 (which is not restricted to a pin-end tool joint) has a base hardness, expressed as Rockwell hardness, in a range from 30-36 HRC, and the localized area 70, after hardening by the process described above, has a localized hardness, expressed as Rockwell hardness, in a range from 56-63 HRC. In certain aspects, the localized hardened area 70 has a localized hardness that is at least 40% greater than the base hardness of the element 72. In certain aspects, the depth of the localized hardened area 70 is in a range from 0.25 in. (0.635 cm) to 0.375 in. (0.9525 cm).

Advantages provided by the disclosed techniques include, without limitation, no welding requirement, shortened process time, reduced machine time, energy competitiveness, lack of need for additional costly filler metals, cracking reduction, distortion reduction, elimination of need for arc gouging once the element is worn, and ease of re-treatment of the element surface or area. It will be appreciated by those skilled in the art that the techniques disclosed herein can be fully automated using conventional equipment configured with software code to perform the techniques as described herein. Aspects of the invention may be implemented with any conventional tools, tubulars, and equipment.

While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims. 

1. A method of protecting an element from excessive surface wear, comprising: exposing a localized area of the element that will be subjected to the excessive surface wear to induction heating for a period sufficient to heat the localized area to an elevated temperature at which the localized area undergoes austenitic transformation; quenching the localized area; tempering the localized area; and cooling the localized area; wherein a result of the method is a localized hardened area formed monolithically with the element and having a localized hardness that is greater than a base hardness of the element.
 2. The method of claim 1, wherein the induction heating is at a frequency in a range from 1 kHz to 10 kHz and a power density of 0.5 kW/in² to 10 kW/in².
 3. The method of claim 1, wherein the element is made of low-alloy steel.
 4. The method of claim 1, wherein a depth of the localized hardened area is at least 0.25 in. (0.635 cm).
 5. The method of claim 1, wherein the tempering step occurs at a temperature in a range from 250° F. to 295° F.
 6. The method of claim 1, wherein the localized hardness, expressed as Rockwell hardness, is in a range from 56 to 63 HRC.
 7. The method of claim 6, wherein the base hardness, expressed as Rockwell hardness, is in a range from 30 to 36 HRC.
 8. The method of claim 1, wherein the localized hardness is at least 40% greater than the base hardness.
 9. The method of claim 1, wherein quenching is in an aqueous polymer medium.
 10. The method of claim 9, wherein the aqueous polymer medium comprises a polymer selected from polyalkylene glycol and polyethylene glycol.
 11. A downhole tool protected from excessive surface wear, comprising: a tool body made of a material having a base hardness; and a localized hardened area formed monolithically with the tool body, the localized hardened area having a localized hardness that is greater than the base hardness.
 12. The downhole tool of claim 11, wherein the localized hardness, expressed as Rockwell hardness, is in a range from 56 to 63 HRC.
 13. The downhole tool of claim 11, wherein the base hardness, expressed as Rockwell hardness, is in a range from 30 to 36 HRC.
 14. The downhole tool of claim 11, wherein the localized hardness is at least 40% greater than the base hardness.
 15. The downhole tool of claim 11, wherein the material of the tool body is comprised of a low-alloy steel.
 16. The downhole tool of claim 11, wherein a depth of the localized hardened area is at least 0.25 in. (0.635 cm). 